1. Field of the Invention
The present invention generally relates to a transfer apparatus, a transfer method, and an image forming apparatus.
2. Description of the Related Art
In an electrostatic recording apparatus such as a printer and a copier employing an electrophotographic method, an image carrier called a photosensitive body is charged uniformly at a prescribed voltage; an electrostatic latent image is formed by exposing the surface of the uniformly charged image carrier based on image information; an image visualizing agent called toner is used to form a visible image (called a toner image); and the formed visible image is transferred and fixed onto a recording medium such as a recording paper to form the image on the recording medium. In a full-color printing apparatus, a process of superposing plural toner images is required. To that end, first, the plural toner images are transferred onto an intermediate transfer medium, and the superposed toner image formed on the intermediate transfer medium is transferred and fixed onto a recording medium.
A continuous-paper printer employing the electrophotographic method has become more and more popular in printing various applications such as direct mail, invoices, and books. As such applications expand, cases where a paper having an uneven surface such as a rough-surfaced paper is used are also increasing. In such cases, unfortunately, defective images are likely to be printed onto such a paper due to poor contact between the photosensitive body and the paper.
Further, in double-sided printing in a continuous-paper printer, a tandem method using two printers is generally used where a paper printed by a first printer is reversed so that the other side of the paper is printed by a second printer. In this case, as well, defective images are likely to be printed when images are transferred onto the paper in the second printer due to unevenness of the surface of the paper caused by the shrinkage of the paper that is heated in a fixing process in the first printer.
As a transfer method capable of providing sufficient contact between the paper and the photosensitive body, a roller transfer method and a belt transfer method are generally known. In the roller transfer method and the belt transfer method, a toner image is transferred onto the paper by using an electrostatic force while the paper is pressed against the photosensitive body by a roller or a belt, respectively. There are two known methods of generating the electrostatic force. One method is to apply current or voltage to the core bar of the transfer roller (hereinafter referred to as “core bar applying method”). The other method is to charge a dielectric layer on the surface of the transfer roller (hereinafter referred to as “external charging method”).
As an example of the “core bar applying method”, Japanese Patent Application Publication No. H07-64411 discloses a transfer bias controlling method for controlling a bias voltage applied to the core bar of the transfer roller. In the method, a voltage is applied to the core bar of the transfer roller so that a bias voltage is formed between the photosensitive body and the transfer roller. To determine an appropriate bias voltage, currents are measured when there is a paper in the transfer nip section and when there is no paper in the transfer nip section. Based on the calculation of the measured data, the bias voltage is controlled. A transfer apparatus employing the “core bar applying method” is generally used in a low-speed or middle-speed printer for cut sheet paper. Unfortunately, it is difficult for a printer employing the “core bar applying method” to provide the necessary stable voltage or current to the rear side of a paper sheet through a roller or a belt for a long continuous period due to changes of circumstance and paper width. Especially, a printer employing the “core bar applying method” is not suited to be used as a continuous-paper printer requiring high throughput and high durability.
On the other hand, in the “external charging method”, a dielectric layer is formed on the elastic layer of the transfer roller, and the surface of the dielectric layer is charged by using a corona electrification device or a roller electrification device. Because of this feature, advantageously, it is not necessary to consider the charge transfer from the inside of the transfer roller, and various materials may be used for the dielectric layer as the surface of the transfer roller. Therefore, an organic material having excellent durability can be selected and used, thereby improving the durability of the transfer roller. Further, when a large-scale printer employing the “core bar applying method” and capable of printing a large paper having an image forming width of 500 mm or more prints on a continuous paper having a short image forming width, a larger area where the transfer roller is not in contact with the paper is generated. In this case, there is a problem that a large current directly flows to a contacting surface between the photosensitive body and the transfer roller, thereby preventing the formation of the electric field necessary for the paper to be printed. However, this problem does not occur in a printer employing the “external charging method”.
As examples of transfer apparatuses employing the “external charging method”, Japanese Patent Publication No. S57-10427 and Japanese Patent Application Publication No. S49-18335 disclose a transfer roller where a corona discharge is used to charge a surface of the transfer roller. Further, Japanese Patent Publication No. S62-3423 discloses a transfer apparatus including a transfer roller having a surface charged by corona discharge, an electrometer measuring the charge amount on the transfer roller, and a discharger for removing electric charges on the surface of the transfer roller so that the charge amount becomes constant and the electric charges on the surface of the transfer roller after charging a recording sheet are removed. Japanese Patent Application Publication No. S51-151544 discloses a transfer apparatus using a transfer belt instead of the above transfer roller and the surface of the transfer belt is charged by corona discharge.
In the “core bar applying method”, the amount of electric charges supplied to the transfer nip section can be easily obtained from the current of a power source electrically connected to the core bar of the transfer roller. Therefore, it is possible to respond to environmental changes including the type of recording paper by appropriately adjusting the voltage or current of the power source. On the other hand, in the “external charging method”, the amount of electric charges supplied to the dielectric layer on the surface of the transfer roller can be detected or adjusted based on the voltage or current of the power source for, for example, a corona charger for corona charging.
However, in a transfer apparatus employing the “external charging method”, the charger electrically charges the dielectric layer of the surface of the transfer roller; the electric charges on the dielectric layer are supplied to the recording paper so as to electrically charge the recording paper when the recording paper is in contact with the transfer roller (transfer nip section); and the toner image is transferred from the photosensitive body onto the recording paper by the voltage difference generated by the supplied electric charges. Because of this feature, it is difficult to directly detect the amount of electric charges supplied from the surface of the transfer roller to the recording paper at the transfer nip section. In addition, the entire charge amount on the surface of the transfer roller is not always supplied to the recording paper. As a result, the charge amount supplied from the surface of the transfer roller to the recording paper at the transfer nip section can not be obtained easily. Because of this difficulty, unfortunately, it is difficult to obtain appropriate transfer conditions responding to environmental changes including a change to a different type of recording paper, thereby degrading the image quality due to, for example, the lowering of transfer efficiency and splashing of toner.